1. Field of the Invention
The present invention relates to a magnetic resonance imaging (MRI) apparatus and an image processing apparatus which excites nuclear spin of an object magnetically with an RF (radio frequency) signal having the Larmor frequency and reconstructs an image based on an NMR (nuclear magnetic resonance) signal generated due to the excitation and, more particularly, to a magnetic resonance imaging apparatus and an image processing apparatus which can obtain a blood flow image in a shorter period of time without a contrast medium.
2. Description of Related Art
Magnetic resonance imaging is an imaging method which excites nuclear spin of an object set in a static magnetic field with an RF signal having the Larmor frequency magnetically and reconstructs an image based on an NMR signal generated due to the excitation.
MRA (Magnetic Resonance Angiography) is known as a technique for obtaining a blood flow image in a field of magnetic resonance imaging. MRA without using contrast medium is called non-contrast-enhanced MRA. In non-contrast-enhanced MRA, a fresh blood imaging (FBI) method is designed which clearly images a blood vessel by acquiring a high-velocity blood flow pumped out from a heart with ECG (electro cardiogram) synchronization (see, for example, Japanese Patent Application (Laid-Open) No. 2000-5144).
Non-contrast-enhanced MRA under the FBI method can obtain an MRA image with arteriovenous separation by obtaining a difference between image data sets acquired by varying a delay time of ECG synchronization. Additionally, the Flow-Spoiled FBI method is designed, which to suppress an artery signal during systole by applying a spoiler pulse in the FBI method. The Flow-Spoiled FBI method images a difference between artery signals in diastole and systole of a myocardium. An ECG-prep scan is designed to decide the optimum delay time of ECG synchronization.
Furthermore, in the FBI method, the Flow-dephasing method is designed to image a low-velocity blood flow, which applies a gradient pulse (Gspoil) in a readout (RO) direction and adds a dephasing pulse or a rephasing pulse to a gradient magnetic field pulse (see, for example, Japanese Patent Application (Laid-Open) No. 2002-200054, Japanese Patent Application (Laid-Open) No. 2003-135430 and U.S. Pat. No. 6,801,800). The Flow-dephasing method can increase relative signal difference between a high-velocity blood flow and a low-velocity blood flow by operation of a dephasing pulse or a rephasing pulse. Then, the relative signal difference allows clearly separating arteries from veins.
This means it is important to enlarge a signal difference between diastole and systole for clear arteriovenous separation. In order to do this, it is necessary to reduce intensity of a signal from a high-velocity blood flow in systole. Therefore, a gradient pulse having an appropriate intensity in an RO direction is set and a blood flow signal from an artery in systole is suppressed by the set gradient pulse. In this state, a blood flow signal in diastole is acquired. Then, subtraction processing and/or maximum intensity projection (MIP) processing is performed to the blood flow signal acquired in diastole and thus only arteries are imaged.
The Flow-prep Scan is designed to perform a preparation scan with changing parameters including an intensity of a dephasing pulse in an RO direction in the Flow-dephasing method (see, for example, Japanese Patent Application (Laid-Open) No. 2003-70766). The Flow-prep Scan makes it possible to obtain the optimum parameter by referencing images acquired while changing a parameter by the preparation scan. A research report about intensity of a dephasing pulse in an RO direction has been made (see, for example, M. Miyazaki, et al., Radiology 227:890-896, 2003).
An echoshare technique with the use of the Half-Fourier method obtains a blood flow image in a shorter scanning time (see, for example, Japanese Patent Application (Laid-Open) No. 2001-149341). Scans in diastole and systole obtain respectively corresponding data sets by this technique. By the scan in systole, only echo data in a low-frequency region important for improving contrast is acquired in a short data acquisition time. Instead, copies of data in a high-frequency region acquired by the scan in diastole are used as data in the high-frequency region that are not acquired in systole. Moreover, data in an insufficient region (even with using a copy of data from diastole) is calculated from each K-space data for diastole and systole by the Half-Fourier method to be used for image reconstruction.
A technique to obtain dynamic information of a blood flow simply without contrast medium and measuring an ECG-synchronization timing by an ECG-prep scan is also known (see, for example, Japanese Patent Application (Laid-Open) No. 2004-329614). This technique uses an ECG-prep scan as an imaging scan. Specifically, as in the case of an ECG-prep scan, dynamic information of a blood flow can be obtained by performing subtraction processing on two-dimensional data sets (acquired more than once by an imaging scan) by changing a delay time gradually from an R wave of an ECG signal.
FIG. 18 is a diagram showing a conventional imaging scan with use of an ECG-prep scan.
In FIG. 18, the abscissa denotes time. As shown in FIG. 18, a trigger signal is set initially at a timing of delay time d1 from an R wave of an ECG signal. Then, a scan for data acquisition is started in synchronization with the trigger signal. Then, a trigger signal is set at a timing of delay time d2 from an R wave of the ECG signal after completion of the scan for data acquisition. Then, a scan for data acquisition is started in synchronization with the trigger signal. In the same way, trigger signals are set at timings of delay times d3, d4, from R waves of the ECG signal respectively, and a scan for data acquisition is started at a different timing in synchronization with each trigger signal. At this time, delay times d1, d2, . . . are set to be changed gradually in a systole of a myocardium showing a great change in velocity of blood flow.
A pulse sequence that consists of a two-dimensional partial FS (flow spoiling) pulse and/or a two-dimensional partial FC (flow compensation) pulse is used as a scan for data acquisition.
The imaging scan as shown in FIG. 18 can reconstruct multiple sets of image data each corresponding to a mutually different delay time from an R wave of the ECG signal.
FIG. 19 is a diagram showing delay times from an R wave of an ECG signal for respective acquisition timings of sets of image data acquired by the imaging scan shown in FIG. 18.
In FIG. 19, the abscissa denotes time and each arrow denotes a timing of an R wave of an ECG signal. As shown in FIG. 19, multiple sets of image data each corresponding to a mutually different delay time from an R wave of the ECG signal are generated by an imaging scan. This means multiple sets of image data each corresponding to a mutually different cardiac time phase are generated. A subtraction image obtained by performing subtraction processing to the sets of image data generated in this way becomes a blood flow image presenting dynamic information of a blood flow. This blood flow image is called the Time-resolved MRDSA (magnetic resonance digital subtraction angiography) image since it is a subtraction image of a blood flow resolved by time.
A technique is also known to obtain dynamic information of a blood flow mentioned above by PI (parallel imaging) which is a high-speed imaging method. PI is a technique for using a PAC (phased array coil) having surface coils as an RF coil for data reception (see, for example, Japanese Patent Application (Laid-Open) No. 2004-329613). By using PI for obtaining dynamic information of a blood flow, scanning time can be reduced.
In the conventional technique for obtaining a Time-resolved MRDSA image, data acquisition time per 1 shot to acquire Time-resolved is MRDSA data used for generating a Time-resolved MRDSA image is considerably long in view of an interval between R waves. Therefore, a TR (repetition time) is set to be about 3RR corresponding to 3 times of a distance RR between R waves.
Consequently, an imaging time of about 60 to 90 seconds is necessary to generate different Time-resolved MRDSA images corresponding to 20 to 30 time phases. Therefore, shortening of imaging time is needed.
In addition to shortening of imaging time, when scanned data is processed simply and appropriately in accordance with the diagnostic purpose and a user can refer to a diagnostic image more easily, working time and diagnostic time of the user can be reduced even if an imaging time might increase.